1. Field of the Invention
This invention relates to pneumatic tires, and more particularly to an improvement of block type or rib.multidot.block type tread patterns in pneumatic radial tires. The specification describes development results on the improvement of traction performance and braking performance in this type of the tire.
2. Related Art Statement
The tread pattern in this type of the tire is particularly block type in many all-season (or so-called all-weather) type tires, wherein each block is always provided with plural slits each having a narrow width to actually close with each other at the ground contact surface during running under loading for exclusively enhancing the traction force on snow. On the other hand, even in block or rib.multidot.block type tread pattern of ordinary tires, substantially the same slits as described above are provided in order to mainly enhance the braking force at a wet road state. Such a slit is usually called a sipe.
The tread pattern having the arrangement of sipes is well-known, so that it is not necessary to present data on such patterns. In general, the usual sipe arrangement is roughly divided into a so-called "blind" type independently of tread grooves comprising circumferential grooves and transverse grooves, "separate" type crossing an island portion of the tread and extending between the circumferential grooves, and "semi-blind" type opening at only one end to the circumferential groove.
The invention is directed to the aforementioned semi-blind type sipe arrangement. Heretofore, this type of the sipe is said to be a compromise between the blind type and the separate type in a point that there is no fear of excessively reducing the rigidity of the island portion in the tread. However, the semi-blind type sipe has still problems that it is insufficient to improve performances on snow in the all-season tire or breaking performances at wet state in the ordinary tire and uneven wear is liable to be caused at the edge of the circumferential groove facing the sipe.